Gold-based alloys have been used for centuries in the manufacture of jewelry. The most common of these are 14-karat compositions, having a traditional yellow color. However, for certain applications, such as diamond settings, a white gold alloy composition is preferred. Traditionally, white jewelry alloys were platinum-based. However, during World War II, platinum was considered to be a strategic industrial metal, and its usage in jewelry items was therefore limited.
A number of elements have been tried to bleach the color of such gold alloy compositions, but, upon information and belief, only alloys containing palladium and nickel have been practical and successful. The main drawbacks of palladium-containing gold alloys are their high cost and high melting temperature. Additionally, these alloys become very soft when exposed to a soldering operation. Nickel, on the other hand, can produce an allergic reaction. Moreover, nickel-containing alloys appear to be prone to stress-corrosion cracking, which may, for example, lead to a prong failure in a diamond setting.
U.S. Pat. No. 3,981,723 discloses a white gold alloy composition consisting essentially of about 50–54% gold, about 27–21% palladium, about 11–16% silver, about 4.5–8% indium and tin, and about 0.5–2.5% iridium or ruthenium. These alloys are largely intended for use in dental applications, have unsuitably high melting temperatures, are costly, and require excessive amounts of palladium.
U.S. Pat. No. 5,372,779 discloses nickel-free white gold alloy compositions containing about 35–50% gold, about 35–63% silver, about 0.0–7% zinc and/or germanium, and less than about 9% palladium. These alloys are apparently intended for 10-karat gold alloy composition applications, and do not appear to provide reversible hardness characteristics.
U.S. Pat. No. 5,635,131 discloses palladium-containing white gold alloy compositions containing about 58.33% gold, about 29% silver and about 2.67% zinc. These alloys do not appear to have the reversible hardness characteristics of the present invention.
U.S. Pat. No. 5,180,551, which is assigned to the assignee of the present application, discloses a number of yellow gold alloy compositions capable of reversible hardness. These alloys contain not less than about 58.03% gold, not less than about 10% silver, not less than about 2% zinc, not less than about 0.02% cobalt, and copper in a weight percent equal to about 100 less the sum total of the weight percent of the gold, silver, zinc and cobalt. The ratio of the copper amount to the silver amount is between about 2.0 to about 3.8. The ratio of the copper amount to the sum total of the silver amount plus twice the zinc amount is between about 1.3 and about 2.5. These compositions have a desirable gold color having a yellow component in the range of about 17.7 to about 20.5 CIE units, and a red component in the range of about 2.6 to about 4.0 CIE units. These compositions are also capable of being selectively hardened to at least 150% of their annealed-hardness values.
Finally, U.S. Pat. No. 5,919,320, which is also assigned to the assignee of the present application, discloses a plurality of nickel-free white gold alloy compositions having reversible hardness characteristics. These compositions consist essentially of about 55–60% gold, about 12–20% silver, about 8–15% copper, 8–18% palladium, about 0.0–1.0% tin, zinc, iridium, or cobalt and, optionally, about 0.005–0.02% iridium and/or ruthenium. The composition may also have about 0.01–0.03% lithium.
Each of foregoing patents is hereby incorporated by reference.
White gold alloy compositions enjoy a fairly large and stable demand in the jewelry industry as less-expensive alternatives to platinum alloys. The most common and inexpensive whitening element that is used in gold alloy compositions is nickel. In a 14-karat alloy, the amount of nickel may vary up to about 15%. Nickel-containing white gold alloy compositions are susceptible to stress corrosion that often leads to failure of the jewelry article. Nickel is also known to cause an allergic reaction when brought into contact with the skin. That is why “nickel-free” has become a requirement for many white gold alloy compositions that are used to make various jewelry articles. Palladium is the only other practical whitening material that can be used in white gold alloy compositions. The concentration of palladium in commercial 14-karat gold alloy compositions may vary up to about 15%. Palladium-containing white gold alloy compositions are typically more expensive than nickel-containing white gold alloy compositions. More over, the palladium-containing compositions typically have a higher melting temperature and are relatively soft.
Most white gold jewelry is plated with rhodium. When so plated, the requirements for white color become more flexible. The Manufacturing Jewelers and Suppliers of America (MJSA), together with the World Gold Counsel (WGC) have developed and adopted three grades of color for white gold alloy compositions based on the yellowness index (“YI”). These are also governed by ASTM D1925. These three grades are as follows:
GradeColor DescriptionYI1PremiumYI < 19.02Standard19.0 < YI < 24.53Off-White14.5 < YI < 32.0
Alloys having a yellowness index of YI>32.0 are considered to be non-white. Jewelry made with grade one or grade two alloys may be used with or without rhodium plating. However, jewelry made with Grade 3 alloys requires rhodium plating.
Accordingly, it would be generally desirable to provide a series of more economical nickel-free white gold alloy compositions, preferably having reversible hardness characteristics.